Calcium oxalate scaling is a major problem in the paper industry, in particular but not exclusively in bleach plants for both kraft and mechanical mills, and in sulfite pulping mills. Calcium oxalate depositions hinder the normal plant operation and decrease the quality of paper produced by increasing the "dirt count". Calcium oxalate originates from oxalic acid present in the wood or formed by oxidation processes during pulping or bleaching; and calcium which can also be present in the wood or enter the system in the process water.
In bleach plants, calcium oxalate scale is typically found on face wires, pumps, pipes, screw presses and down stream process equipment. In sulfite mills the most common calcium oxalate scaling sites are evaporators and liquor handling equipment.
U.S. Pat. No. 4,872,995 to Chen et al., discloses methods of controlling the formation of calcium oxalate in aqueous systems in which (meth)acrylic acid/allyl ether copolymers are employed. The use of poly(epoxysuccinic) acid to inhibit scale formation, including calcium scale, is disclosed in U.S. Pat. No. 5,062,962 to Brown et al.
Canadian Patent Application No. 2,005,982 discloses a process of inhibiting the formation of calcium carbonate scale in wood pulp production comprising adding to the pulp, a hydrolyzed copolymer of maleic anhydride with a monoethylenically unsaturated monomer or a mixture of monomers provided that the copolymer is other than a maleic acid/(meth)acrylic acid copolymer. The preferred copolymer is a hydrolyzed copolymer of (A) maleic anhydride with (B) at least one monoethylenically unsaturated monomer other than acrylic or methacrylic acid or with a mixture of (B) and (C) acrylic acid or methacrylic acid. The copolymer is added to the pulp liquor.
U.S. Pat. No. 5,015,390 discloses a method for controlling scale formation in acqueous systems comprising treating the system with a water soluble polymer comprising an ethylenically unsaturated dibasic carboxylic acid or anhydride preferably maleic acid or anhydride and at least One quaternary dialkyldiallyl ammonium monomer preferably dimethyldiallyl ammonium chloride.
It was discovered that traditional inhibitors for calcium carbonate, such as phosphonates, polyacrylates and polyphosphates have only a modest effect on calcium oxalate control. In general, the inhibiting effect of a compound is specific for a sparingly soluble salt as this effect depends on structure compatibility between compound and crystal.